Cryoelectric device



Jan. 31, 1967 c, M, WINE CRYOELECTRIC DEVICE Filed Aug. 19, 1964 a L 7mam M NW 7. x my w\flw 1 2 my, 7. r 2 W m 1 D" I R I mw P x WW7 M w. M4. W 4 0 E 4 WMNW M5 Ww; WW 2 00 N Z 5 Wm W in w o x M R m w 2 W 5 aUnited States Patent G 3,302,152 CRYOELECTRIC DEVICE Charles M. Wine,Princeton, N.J., assignor to Radio Corporation of America, a corporationof Delaware Filed Aug. 19, 1964, Ser. No. 390,560 Claims. (Cl. 338-32)plane in close proximity to the line substantially reduces theinductance of the line, a desirable feature in many circuitapplications. The strip line may be formed by vacuum depositing a metal,such as tin or lead, through a mask onto a substrate (the insulatedsurface). So

made, the line is generally found not to be of uniform cross-section.Its edges taper, that is, they are thinner than the center portion ofthe line. This is believed to be due, among other things, to penumbraeffects during the vapor deposition.

If one assumes the current carried by a superconductor line todistribute uniformly along the width of the line, the current density atthe edges of a line such as discussed above is greater than that at thecenter of the line. Due to this non-uniform current density, at somevalue of current at which the center portion of the line remainssuperconducting, the current density at the edges of the line exceedsthe critical value, that is, it becomes sufiicient to drive the edges tothe normal (resistive) condition. Once the edges are driven normal, thenormal region rapidly spreads to the remainder of the line. Therefore,it is ordinarily necessary to operate the line at a lower level ofcurrentone at which the current density at the edges is less than thecritical value, than the bulk of the line can support.

The so-called edge effects discussed above are largely avoided accordingto the present invention by spacing the edges of the line further fromthe ground plane than the center portion of the line as, for example, byplacing insulator strips beneath these edges. This substantiallyincreases the inductance of the line at its edges, and consequentlyreduces the density of current flow at the edges.

The invention is discussed in greater detail below and is illustrated inthe following drawings, of which:

FIGURE 1 is a plan view of a prior art in-line cryotron;

FIGURE 2 is a cross-section taken along line 2-2 of FIGURE 1;

FIGURE 3 is a cross-section through an in-line cryotron according to thepresent invention; and

FIGURE 4 is 'a graph illustrating the performance of the prior artcryotron as contrasted to the one in the present invention.

In the discussion which follows, a low-temperature environment, such asa few degrees Kelvin, at which superconductivity is possible, isassumed.

The cryotron of FIGURES 1 and 2 includes a ground plane 10, which isformed of a superconductor, such as lead, and a thin insulating film 12,which may be formed of silicon monoxide, located on the ground plane. Agate element 14 is located over the insulated ground plane and a controlelement 16 is located over the gate element. The gate element may beformed of a superconductor, such as tin, and the control element may beformed of a superconductor, such as lead. The two elements are insulatedfrom one 'another by an insulating layer 18, such as silicon monoxide.

The gate element 14 is for the purpose of carrying a current to someload such as a drive line of a superconductor memory. In one condition,the gate element is superconducting and offers zero resistance to thisflow of current. In another condition, current is passed through thecontrol element 16 to produce a magnetic field of sufficient magnitudeto drive the gate element from its superconducting to its normal(resistive) condition. In this second condition, the gate elementexhibits a finite resistance to the flow of current.

The gate element 14 is normally fabricated by vacuum deposition througha mask. When made in this way, the edges 20, 20a taper rather than beingperpendicular to the insulator surface. A current passed through thegate element 14 tends to distribute uniformly over the width w of thegate element. Since the edges 20 and 20a are thinner than the remainderof the gate element, the current density at these edges tends to begreater than the current density within the remainder of the gateelement. As a result of this non-uniform current density, the edges 20and 20a tend to be driven normal at a value of current which theremainder of the gate element could otherwise support while in thesuperconductive state. As is well understood, these normal areas tend tospread to the remainder of the gate element, driving the entire gateelement normal and limiting the current-carrying capacity of the gateelement.

In the embodiment of the invention illustrated, strips of insulatingmaterial 22 and 22a, shown in FIGURE 3, are laid down on the insulator12 prior to the time that the gate element is formed. The gate elementis vacuum deposited onto the layer 12 with the edges 20 and 20a thereofon the insulator strips 22 and 2211. Thus, the edges 20 and 20a arespaced from the ground plane 10 a substantial distance greater than thebulk 24 of the gate element is spaced from the ground plane. Theinductance of the gate element is 'a function of its spacing from theground plane 10. Accordingly, the inductance of the edges 20 and 20a ofthe gate element is substantially greater than that of the remainder ofthe gate element.

As is well understood in the cryoelectric art, if a current is appliedto two superconductors in parallel, the current divides in inverseproportion to the inductance of the two conductors. In the presentinstance, the conductors which exhibit a high inductance can beconsidered to be the edge portions 20 and 20a, and the conductorexhibiting a low inductance can be considered to be the center region.Accordingly, when a current is applied to the gate element, it tends todistribute so that less of the current flows at the edges 29 and 20athan in the bulk 24. Therefore, even though the gate element is thinnerat its edges 20 and 20a and the current density accordingly tends to begreater there, the increased inductance at the edges reduces the amountof current flowing at these edges and tends to reduce the currentdensity. The over-all result is that current flow through the gateelement in the arrangement of FIGURE 3 is of more uniform densitythrough out the width of the gate element than in the arrangement ofFIGURE 2.

In plan view, the cryotron of FIGURE 3 has substantially the sameappearance as the one shown in FIGURE 1.

Typical dimensions of an in-line cryotron such as shown in FIGURE 3 are:

Gate element 14:

width-l0 mils film thickness5000 A. (Angstroms) Control element 16:width-15 mils film thickness5000 A. insulation layer thickness800() A.Insulation strips 22 and 22a:

width5 mils. (These strips were overlapped by the edges of the gateelement over an 'area 2 mils wide.) thickness-8000 A.

The principles of the invention are applicable not only to in-linecryotrons, but also to strip transmission lines, crossed-film cryotrons,and so on. A crossed-film cryo-- tron is one in which the gate elementextends at right an gles to the control element. As in the case ofin-line cryo-- trons, the gate element may be formed of a superconductorsuch as tin, and the control element may be formed. of a superconductorsuch 'as lead.

To check the operation discussed above, four crossed-- film cryotrons onground planes were fabricated. A plot: of the resistance of the cryotronvs. the temperature (the temperature being the factor varied) is shownin FIGURE- 4. Two of the cryotrons were made with insulation stripsunder the edges of the gate electrode to space the: edges further fromthe ground plane than the center region. of the gate element similarlyto what is shown in FIG-- URE 3, and two were conventional cryotrons inwhich the gate element was uniformly spaced from the ground. plane.Except for the insulation under the edges of the: gate elements in twoof the cryotrons, they were in otherrespects substantially identical.The significant improve-- ment in performance obtained with thecrossed-film cryotrons of the present invention is believed to beself-evident from the curves.

As the cryotrons built were tested individually (rather than in treeconfigurations), temperature was made thevariable and resistance wasmeasured by passing a rela-- tively small current through the gateelement. This was to prevent run-away effects due to heating of the gateelements. However, as is well understood in this art, simi-- lar curvesmay be obtained with the temperature maintained constant at some value,such as 3 K. or 3 /2 K. and current employed as the variable or runningparameter, providing these temperature effects are elimi nated orcompensated for.

What is claimed is: 1. A two-conductor transmission circuit comprising:a superconductor ground plane; and

a thin-film superconductor strip line lying over and insulated from theground plane both at its center and at its longer edges and exhibiting asubstantially greater inductance at its opposite edges, by virtue ofincreased spacing at said edges from the ground plane, than at itscenter portion.

2. A two conductor transmission circuit comprising:

a superconductor ground plane;

insulation on one surface of the ground plane which is substantiallythicker in certain regions thereof over the ground plane than in otherregions thereof over the ground plane, and

a thin film superconductor strip which is thinner at its opposite edgesthan at its center lying on said insulation, the opposite edges of saidstrip lying on the thicker regions of the insulation and the centerportions of the strip lying on the thinner region of said insulationwhereby the strip exhibits a substantially greater inductance at itsopposite edges than at its center portion.

3. A transmission circuit comprising:

a superconductor ground plane;

insulation on one surface of the ground plane, two spaced strips ofwhich are substantially thicker than other portions of the insulation onthe ground plane, said two spaced strips of insulation and theinsulation between these strips lying over the ground plane; and

a thin film superconductor line which is thinner at its opposite edgesthan at its center lying on the insulation, the edges of said line lyingon said thicker insulation strips and the center portion of said linelying on the relatively thinner portion of the insualtion, whereby therelatively thinner edges of said line are spaced substantially furtherfrom the ground plane than the center portion of the line.

4. A transmission circuit comprising, in combination:

a superconductor ground plane;

insulation on one surface of the ground plane;

a thin film superconductor line on said insulation, said line beingthinner at its edges than at its center; and

additional insulation over the ground plane lying under the edges of theline spacing said edges a substantially greater distance from the groundplane than the center portion of the line.

5. A cryotron comprising:

a superconductor ground plane;

insulation on said ground plane;

a thin film superconductor gate element insulated from and lying overthe ground plane, the element being thinner at its edges than at itscenter;

additional insulation under the edges of said gate element for spacingsaid edges substantially further from the ground plane than the centerportion of the gate element; and

a thin film superconductor control element lying over and insulated fromthe gate element.

References Cited by the Examiner UNITED STATES PATENTS 2,989,714 6/1961Park et al. 338-32 2,989,716 6/1961 Brennemann et al. 340-173 X3,059,196 10/1962 Lentz 33832 3,098,967 7/1963 Keck 338--32 X 3,100,2678/1963 Crowe 33832 X 3,191,055 6/1965 Swihart et al 307-885 3,207,9219/1965 Ahrons 340173 X 3,233,199 2/1966 Hagedorn 338-32 3,234,439 2/1966Alphonse 33832 OTHER REFERENCES Superconductive Computers by ThomasMaguire, Electronics, November 24, 1961, pages 45, '51.

IBM Technical Disclosure Bulletin, by R. E. Jones, Jr, vol. 7, N0. 3,August 1964, pages 269-270.

RICHARD M. WOOD, Primary Examiner,

W. D. BROOKS, Assistant Examiner.

5. A CRYOTRON COMPRISING: A SUPERCONDUCTOR GROUND PLANE; INSULATION ONSAID GROUND PLANE; A THIN FILM SUPERCONDUCTOR GATE ELEMENT INSULATEDFROM AND LYING OVER THE GROUND PLANE, THE ELEMENT BEING THINNER AT ITSEDGES THAN AT ITS CENTER; ADDITIONAL INSULATION UNDER THE EDGES OF SAIDGATE ELEMENT FOR SPACING SAID EDGES SUBSTANTIALLY FURTHER FROM THEGROUND PLANE THAN THE CENTER PORTION OF THE GATE ELEMENT; AND A THINFILM SUPERCONDUCTOR CONTROL ELEMENT LYING OVER AND INSULATED FROM THEGATE ELEMENT.